1. Field of the Invention
The present invention relates to an apparatus for polishing substrates. More particularly, the invention relates to a platen/polishing pad assembly having a compliant surface to improve polishing uniformity of substrates.
2. Background of the Related Art
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited and removed from a substrate during the fabrication process. Often it is necessary to polish a surface of a substrate to remove high topography, surface defects, scratches or embedded particles. One common polishing process is known as chemical mechanical polishing (CMP) and is used to improve the quality and reliability of the electronic devices formed on the substrate.
Typically, the polishing process involves the introduction of a chemical slurry during the polishing process to facilitate higher removal rates and selectivity between films on the substrate surface. In general, the polishing process involves holding a substrate against a polishing pad under controlled pressure, temperature and rotational speed (velocity) of the pad in the presence of the slurry or other fluid medium. One polishing system that is used to perform CMP is the Mirra.RTM. CMP System available from Applied Materials, Inc., and shown and described in U.S. Pat. No. 5,738,574, entitled, "Continuous Processing System for Chemical Mechanical Polishing," the entirety of which is incorporated herein by reference.
An important goal of CMP is achieving uniform planarity of the substrate surface. Uniform planarity includes the uniform removal of material from the surface of substrates as well as removing non-uniform layers which have been deposited on the substrate. Successful CMP also requires process repeatability from one substrate to the next. Thus, uniformity must be achieved not only for a single substrate, but also for a series of substrates processed in a batch.
Substrate planarity is dictated, to a large extent, by the construction of the CMP apparatus and the composition of the consumables such as slurry and pads. In particular, a preferred construction allows for a proper balance between rigidity (or stiffness) and compliance (or flexibility) of the polishing device, and in particular to the stiffness and compliance of the polishing pad. In general, stiffness is needed to ensure within-die uniformity while sufficient compliance provides within-substrate uniformity. Within-substrate uniformity refers to the ability of the CMP apparatus to remove features across the diameter of the substrate regardless of substrate shape and/or topography across its surface. Within-die uniformity refers to the ability of the CMP apparatus to remove features within a die, regardless of size and feature density.
Conventional polishing systems typically include a platen having a polishing pad disposed thereon. Current state of the art strongly suggests the use of more than one polishing pad to provide compliance to the pad for improved results both within-substrate and within-die. For example, two pads are typically assembled together into a stack, which may be termed a "composite polishing pad." The composite pad usually includes combination of a rigid pad and a compliant pad. A typical polishing apparatus 10 comprising a metal platen 12 having a composite polishing pad 14 mounted thereto is shown in FIG. 1. Both the composite polishing pad 14 and the platen 12 are generally disc-shaped and of equal diameters. The top (upper) pad 16, is brought into contact with a substrate to perform the polishing process, while the bottom (lower) pad 18 is secured to a smooth upper mounting surface of the rotatable platen 12 to provide a seating surface for the top pad 16. An adhesive 20, such as a pressure sensitive adhesive (PSA) is provided on the back face of the pads 16, 18 to bond the pads to one another and to the platen 12. The top pad 16 is typically made of cast polyurethane while the bottom pad 18 is typically made of polyester felt stiffened with polyurethane resin. Other pads having different material composition are also available and known in the industry.
Generally, it is preferable that the top pad 16 be stiffer than the more compliant bottom pad 18 to provide a sufficiently rigid polishing surface. Typically, stiffness provides better within-die uniformity, while some compliance is needed to ensure within-substrate uniformity. The combination of pads having the proper proportions of stiffness and flexibility can achieve good planarity and uniformity over the surface of the substrate. In addition, the polishing profile on a substrate can be changed or modified by changing the thickness of either or both of the upper and lower pads. The change in thickness without a change in composition can change the properties of the composite pad in terms of stiffness and compliance.
However, a number of problems are associated with the conventional composite, or stacked, pad construction. One problem with composite pads is the interdependence of the individual pads upon one another. For example, a pressure exerted on the upper pad is transmitted to the lower pad. Because the upper pad is generally a rigid material having limited compressibility, the upper pad accommodates the pressure by translation, or displacement, of its position. Consequently, the lower pad experiences a pressure due to the deflection of the upper pad. The pressure on the lower pad is absorbed by compression of the lower pad. The total compressed volume of the lower pad depends at least partially on the compressibility of the material. However, because the compression cannot be completely localized to the origin of the pressure, the lower pad will experience deformation around the perimeter of the applied pressure. In the case of a shearing force, such deformation can result in ripples or waves on the lower pad due to the mass compression and redistribution of the lower pad, much like the effect of a shearing force applied to a carpet or rug. During operation, the waves exert a resultant force on the upper pad which can result in non-uniform polishing and undermines the goal of substrate planarization.
Another problem with composite pads is that each additional layer, e.g., pad and adhesive layer, in the stack acts as a source of variation affecting the overall stiffness, compression and/or compliance of the stack. The greater the number of layers or even variations in the thickness of pads, the greater the potential for variation. As a result, a polishing device utilizing a composite polishing pad is often unable to achieve desired polishing results over a number of substrates. Specifically, variation in compressibility, loss of within-substrate uniformity, uncontrolled wetting of the lower pad, and variation from pad to pad result due to multiple process variables. In addition, the planarity changes as the top pad is worn away by a process known as conditioning the pad. As the top pad is reduced in thickness, the planarity may decrease with increasing numbers of substrates polished on the pad.
One solution has been to minimize the number of layers in the composite polishing pad. Thus, the goal in CMP would be to remove the bottom pad and secure the top pad directly to the upper surface of the platen. Removal of the bottom pad also eliminates the need for one layer of the adhesive. However, it has been discovered that elimination of the bottom pad and mounting the polishing pad directly on the platen results in an overly rigid pad/platen assembly which compromises the compliance of the assembly. The rigidity is a consequence of directly interfacing the rigid top pad with the non-compliant platen surface, typically made of aluminum, ceramic, granite or other materials.
Therefore, there is a need for a platen/pad assembly which eliminates the problems of conventional bottom pads while providing sufficient compliance and rigidity during polishing.